I'm not sure how Blizzard will make an entire expansion of it, but we can worry about that bridge when people are whining a week in with no content. In the meantime, I want to address the panda-haters.
You think they look silly, are a joke, and do not at all fit in? I agree. You know what else looks silly and is a joke? Gnomes. Incidentally, the biggest problem with gnomes is their inability to be druids and turn into even more cute bears. Let's all picture a gnome bear form and say daw. Dawwww! So cute.
Warcraft is a flexible franchise. It isn't quite clear how it does it, much like the typical super-power battle in anime where everything is driven by shouting and angry faces. Somehow it works. It will be weird for a while. I don't recall having a particularly positive reaction to draenei, especially with the retcon, but they work. They fit in now. Maybe it took some mental rearranging on my part, but it happened and now draenei are just fine. Don't worry about whether a race fits in; it will, eventually.
Maybe the lore will be awful. Maybe it will be great. Maybe the starting area will be the greatest or worst thing ever. We don't know. My assumption is that it will be like most of WoW: fairly fun in general, but possibly hit-or-miss in terms of any sort of emotional reaction, regardless of the 'legitimacy' of the race in question. Just look at my thoughts on the starting areas recently. The gnome story was a good one, but the initial area was just terrible. In contrast, the orc story wasn't particularly great in any way, but it sort of worked out into a bit of fun, and I still got to beat sleeping peons. My prediction is that if you play the pandaren starting area, when you finish you will not think "what a waste of time", and will instead be annoyed that you got dumped into some boring nelf area, like they did with the worgen.
I'm not suggesting that the expansion will be worth your money, or a sub either. I think the expansion will be good and the added content will be good, but it may very well not be good enough. What is good enough? That's something for you to decide. For example, I think that Coke is a good drink; it tastes good, it has sugar and caffeine that make me nostalgic for my hyper younger days, and on top of that it's very cheap. But the health cost, and low but still relevant financial cost, mean that I don't drink it very often at all. It is good, but not good enough.
Speaking of sugary water, did the Mountain Dew tie-in ruin WoW for anyone? It sure ruined it for me, for a very short time, and then I went back to doing whatever it was I was doing and complaining about my usual things. See? Warcraft is flexible and durable. It can withstand just about anything. It's like some mutated slime that devours all, turning it into a homogeneous goo of rat genocide. Maybe you don't like slimy rat genocide, but I'm not sure a few pandas are going to make it any worse.